1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a MESFET (Metal-Semiconductor Field Effect Transistor) type semiconductor device.
2. Description of the Related Art
As mobile telephone and personal "handyphone" telephone systems (PHS) have recently become popular, low cost and compact radio apparatus for portable phones and base stations have become desirable. Conventional n-channel MESFETs in such devices are used in a depletion mode so that it is necessary to apply a positive bias to the drain electrode and a negative bias to the gate electrode. Therefore, positive and negative power sources are required, which results in an increase in size and weight of the devices.
In this context, it is desireable to replace depletion mode MESFETs with enhancement mode MESFETs, which do not require application of a negative bias to the gate electrode.
An enhancement mode MESFET can be achieved by depleting the whole thickness of an electron transfer layer under the gate electrode, under a condition where no voltage is applied to the gate electrode. If the electron transfer layer is thinned, however, the sheet resistance increases, and therefore the gain and output power lowers. Furthermore, as a positive voltage is applied during the enhancement operation, a gate leak current increases so that a large saturation drain current is impossible and the output power is lowered.
Because of these significant problems, enhancement mode MESFETs have not been put into practical use.